1. Field of the Invention
The present invention relates to a Schottky barrier diode comprising a barrier metal between a wire-bonded electrode and a semiconductor substrate.
2. Description of the Background Art
FIG. 8 is a cross-sectional view of a conventional rectifier diode. In a Si substrate, a P anode layer 205 is formed on the upper surface of an N− layer 202 and an anode electrode 203 is formed on the upper surface of the P anode layer 205. An N+ cathode layer 201 is formed on the rear surface of the N− layer 202 and a cathode electrode 204 is formed on the rear surface of the N+ cathode layer 201.
FIG. 9 shows changes of current with respect to time when the diode of FIG. 8 is switched from its on state to its off state, i.e., during reverse recovery of the diode. Where IF is the value of steady-state current and IRR is the minimum value of current. The time during which a reverse current flows is referred to as a reverse recovery time TRR. A gently-sloping recovery of current from IRR to zero is referred to as soft recovery. Although not shown, a reverse voltage is applied to the diode during reverse recovery, and the product of reverse voltage and current is referred to as recovery loss.
In general, it is desirable for rectifier diodes to have small steady-state loss which is the product of forward voltage and current during the on state, to have small recovery loss, and to provide a soft recovery characteristic.
For the above requirements, a variety of diode structures have been suggested. A structure shown in FIG. 10 has been suggested in the Proceedings of the 1987 IEEE International Electron Devices Meeting, pp. 658-661, 1987. In a diode 200 of this structure, the P anode layers 205 are selectively formed in the upper main surface of the N− layer 202 and Schottky junction regions 206 are formed in upper regions of the N− layer 202 where the P anode layers 205 are not formed. The anode electrode 203 is formed on the upper surfaces of the P anode layers 205 and the Schottky junction regions 206. The P anode layers 205 and the anode electrode 203 are in ohmic contact. The N+ cathode layer 201 is formed on the rear surface of the N− layer 202 and the cathode electrode 204 is formed on the rear surface of the N+ cathode layer 201.
Now, the operation of the diode 200 with the structure of FIG. 10 will be described. When a forward-biased anode voltage is applied between the anode electrode 203 and the cathode electrode 204 and increased beyond a predetermined threshold value, the Schottky junction regions 206 first enter the on state. The threshold voltage at this time varies depending on the barrier height of the Schottky junction, and it is lower than that which causes pn junction regions to enter the on state. Further, few holes are injected from the Schottky junction regions 206 into the N− layer 202. If the anode voltage is further increased, hole injection from the P anode layers 205 into the N− layer 202 starts and also causes pn junction regions to enter the on state. The diode 200 has few holes injected from the Schottky junction regions 206 into the N− layer 202, and correspondingly, its carrier concentration in the vicinity of the anode is lower than that in a diode formed of only pn junctions. The value IRR at the reverse recovery is determined by the carrier concentration in the vicinity of the anode. In the diode 200 of FIG. 10, a low carrier concentration in the vicinity of the anode results in a small IRR, thereby reducing recovery loss and providing a soft recovery characteristic. That is, a reverse recovery characteristic of the diode is improved. The diode 200 can also relax electric fields applied across the Schottky junction regions 206 and can increase a reverse breakdown voltage by providing a reach-through of a depletion layer which extends from the selectively formed P anode layers 205 to the Schottky junction regions 206.
A diode disclosed in Japanese Patent Application Laid-open No. 58-60577 (1983) and shown in FIG. 11 has a structure in which the Schottky junction regions 206 in the diode of FIG. 10 are replaced by P anode layers 207 which are thin and have a low impurity concentration. This diode has no Schottky junction regions formed therein and thus the value IRR is not as small as that for the diode 200. Accordingly, the reverse recovery characteristic is not so improved.
A diode disclosed in Japanese Patent No. 2590284 and shown in FIG. 12 has a structure in which the P anode layers 207 in the diode of FIG. 11 are replaced by P anode layers 208 which are thinner than the P anode layers 207. The P anode layers 208 are formed by diffusing a P-type impurity contained in the anode electrode 203. This diode provides a Schottky characteristic for a forward voltage since the Schottky junction regions are formed in the upper main surface of the P anode layers 208.
A diode disclosed in Japanese Patent Application Laid-open No. 6-196723 (1994) and shown in FIG. 13 has been proposed by the inventor of the present invention who aimed to form Schottky junction regions 210 in upper regions of the N− layer 202 where the P anode layers 205 are not formed, by forming a barrier metal 209 such as TiW between the Si substrate and the anode electrode 203. However, later research has shown that the Schottky junction regions 210 cannot be formed in the N− layer 202 by merely forming the barrier metal 209 of TiW on the upper surface of the Si substrate.
According to the teaching of Japanese Patent No. 2590284, when wires are bonded onto the anode electrode 203, the diode 200 of FIG. 10 as a power device has defects caused by pressure at the interface between the anode electrode 203 and the Si substrate. Since electrons in the conduction band flow into those defects, leakage current increases with the application of a reverse bias voltage. This produces a problem of reducing the breakdown voltage.
The diode of FIG. 11 has no Schottky junction regions formed therein and is thus inferior in the reverse recovery characteristic to the diode 200 of FIG. 10.
The diode of FIG. 12, although having the Schottky junction regions formed therein, allows only a narrow margin for structure because of the very small thickness of the P anode layers 208 and is correspondingly difficult to manufacture.
The diode of FIG. 13 has no Schottky junction regions 210 formed therein when TiW is used for the barrier metal 209 formed on the upper surface of the Si substrate, and thus has a poor reverse recovery characteristic. The effectiveness of the barrier metal 209 for defects occurring in wire bonding has not been suggested in Japanese Patent Application Laid-Open No. 6-196723 (1994).
Japanese Patent Application Laid-open No. 2000-261004 has also disclosed a diode having a similar structure to that of FIG. 13, but it has not suggested the effectiveness of a barrier metal for defects occurring in wire bonding.